It is very widely acknowledged that the natural resources in the subsoil of seas, oceans, and large lakes are potentially very significant.
However, in some cases, these resources remain undetected or unexploited due to the technical obstacles created by the presence of large depths of water.
Direct or drilling prospecting methods, which are frequent within the scope of terrestrial prospecting, clearly prove to be difficult to transfer to aquatic environments if the technical requirements and costs associated with the use of such methods are taken into consideration.
For this reason, indirect prospecting methods are generally used, making it possible to deduce the geological nature of the subsoil on the basis of the measurement of a physical value.
A first conventional indirect prospecting method for the exploration of natural subsoil resources in aquatic environments uses a measurement of the propagation of a seismic wave, obtained for example by means of the deflagration of an explosive charge under the water surface, after reflection thereof on the various strata of the subsoil of the aquatic environment. However, the interpretation of the results obtained using this method remains difficult in some geological contexts, i.e. for example in the presence of salt domes, thick basalt flows or very complex geological structures.
The current trend is thus to generalise the use of a second indirect prospecting method by means of the passive magnetotelluric measurement of the electrical conductivity of the subsoil under the stretch of water. This method is referred to as the magnetotelluric method (MT). Known and used for many years to evaluate natural resources in terrestrial subsoil, it was only recently implemented in an aquatic environment, particularly on the continental shelf in a marine environment, due to the development of data acquisition and processing means. In addition, it offers the advantage of not having any impact on the environment as it is a passive method.
The electrical conductivity of the soil varies with the mineralogical nature of the rocks, and depends among other things on the temperature and pressure. It is also very sensitive to the presence of conductive inclusions such as hydrothermal water, brine associated with oil deposits, metals and metallic salts in sulphides form for example, and also the degree of connectivity of these inclusions. In this way, a measurement of the electrical conductivity makes it possible to identify the geological units and the relative porosity of the constituent rocks.
This method uses the measurement of the natural fluctuations of horizontal geoelectric and geomagnetic fields to determine a surface impedance. This impedance, referred to as the magnetotelluric impedance, obtained on a wide range of frequencies, makes it possible therefore to determine the distribution of the electrical conductivity under the soil surface.
It is observed that, in order to obtain suitable mapping of the geographic strata in the soil, it is possible to complete the information from the electrical conductivity measurement with other interpretations based on seismic or gravimetric data.
One problem encountered with this method is that, in seawater, frequencies greater than a few thousandths of Hertz are rapidly attenuated with the distance or depth. This results in a considerable loss in the power of the electrical field or magnetic field for these frequencies. Consequently, as soon as one moves away from the surface of the seabed, it rapidly becomes impossible to make an acceptable measurement in the frequency range (0.01 to 100 Hertz) useful for the interpretation of the nature of the geographic strata.
A first solution to this problem is presented in the document U.S. Pat. No. 5,770,945. This document proposes to make a measurement of the horizontal components of the natural and magnetic fields using an autonomous instrument resting on the seabed and comprising at least two magnetic sensors.
A first drawback of this technical solution is that it only measures the horizontal components of the electric and magnetic fields, which does not make it possible to describe subsoils with complex compositions accurately.
In order to remedy this drawback, the document WO 03/104844 proposes to equip an instrument according to the document U.S. Pat. No. 5,770,945 with a vertical mast in which two electrodes intended to measure the vertical components of the electric field are placed. However, this technical solution remains difficult to perform in an appropriate manner as the vertical component of the electric field has a low intensity and is difficult to measure on the seabed. Moreover, such a measurement is necessarily disturbed by the instability of the position of the sensor.
A second drawback of the technical solution proposed by U.S. Pat. No. 5,770,945 is the sensitivity of the sensors and subsequently of the instrument to sea currents present on the seabed.
A third drawback is due to the type of sensor used, which is very bulky and heavy, which results in excessive instrument weight and size, which also makes it more sensitive to sea currents.